Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead moves back and forth across the page shooting drops as it moves. Typically, a service station is mounted within the printer chassis to clean and protect the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting." The waste ink is collected in a reservoir portion of the service station, which is often referred to as a "spittoon."
For storage, or during non-printing periods, the service stations usually include a capping system which humidically seals the printhead nozzles from contaminants and drying. Typically, the cap is an elastomeric enclosure having sealing lips which surround the nozzles and form an air-tight seal at the printhead face. Usually these caps include a venting feature that is used during capping to avoid forcing air into the nozzles, which would result in de-priming the nozzles. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead.
After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that have collected on the printhead. These wipers were typically made of an elastomeric material, for instance a nitrile rubber, ethylene polypropylene diene monomer (EPDM) elastomer, or other types of rubber-like materials. The wiping action is usually achieved by either moving the printhead across the wiper, or moving the wiper across the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. For example, to provide faster, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solid content than the earlier dye based inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper.
Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids contained within the new pigment based inks. Ink residue also accumulates on the printhead face from excessive ink aerosol or over spray, particularly with the pigment based inks. After drying, this ink is difficult to remove, and if left on the pen face contributes to poor pen performance. For example, partially or totally blocked or occluded nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality.
Another characteristic of the new pigment based inks contributes to the nozzle clogging problem. The pigment based inks use a dispersant to keep the pigment particles from flocculating. Unfortunately, the dispersant tends to form a tough film on the printhead face as the ink vehicle evaporates. Besides the debris accumulated on the printhead face from ink over spray, paper crashes and servicing, this dispersant film also attracts paper dust and other contaminants. The dispersant film on the printhead face, as well as ink residue and debris surrounding the nozzles, is quite difficult to remove from the printhead.
With the earlier dye based inks, basically only the wiper blades were used to clean the printhead face. Unfortunately, the tough film formed by the pigment dispersant is not easily removed by these elastomeric wipers. Instead, this residue tended to ball up and roll, in a manner similar to the way that the adhesive known as rubber cement balls up when dried.
Several wet wiping systems have been proposed that wet the printhead then wipe it while still wet. One type of system spits ink then immediately wipes the ink from the printhead. Another system spits ink on the wiper then wipes the printhead with the wet wiper. Both of these ink-wiping systems used an EPDM elastomeric wiper. Another type of system applies a solvent to the printhead. In this system, the solvent is supplied through a saturated applicator to the printhead using a capillary or wicking action. The solvent is then wiped from the printhead using an EPDM elastomeric wiper. This solvent based wiping system unfortunately adds complexity and cost to the overall product.
Thus, a need exists for an improved inkjet printhead servicing system, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.